A simple excersice of validating a form with vanilla JS.

form-validation.js

/**
<form>
  <input type="radio" id="type_person" name="type" value="person" checked/>
  <input type="radio" id="type_company" name="type" value="company"/>
  <input type="text" id="first_name" name="first_name" value="John"/>
  <input type="text" id="last_name" name="last_name" value="Doe"/>
  <input type="text" id="email" name="email" value="[email protected]"/>
  <input type="text" id="company_name" name="company_name" value=""/>
  <input type="text" id="phone" name="phone" value="234-567-890"/>
</form>
**/

const solution = () => {
  const personFields = [
    { id: "first_name", rule: "not-empty" },
    { id: "last_name", rule: "not-empty" },
    { id: "email", rule: "email-address" }
  ];

  const companyFields = [
    { id: "company_name", rule: "not-empty" },
    { id: "phone", rule: "phone-number" }
  ];

  const typePerson = document.querySelector("#type_person");
  const typeCompany = document.querySelector("#type_company");

  if (typePerson.checked && !typeCompany.checked) {
    // Validate first_name, last_name & email.
    return validateFields(personFields).allValid;
  } else {
    // Validate company_name & phone.
    return validateFields(companyFields).allValid;
  }
};

/**
 * Given an array of fields, return true if all the fields are valid.
 * It also does returns the fields that were checked to allow for later displaying errors to the user.
 *
 * @param {Array<Object>} fields - An array containing fields as object (ie. `{ id: '', rule: '' }`).
 * @returns {Array<Object>} Returns an array of object with results for every fields validated.
 */
const validateFields = fields => {
  const fieldsResults = {
    fields: [],
    allValid: false,
    valid: 0,
    invalid: 0
  };

  for (let i = 0, l = fields.length; i < l; i++) {
    const field = fields[i];
    const el = document.querySelector(`#${field.id}`);
    const isValid = validateRule(field.rule, el.value);
    fieldsResults.fields.push({
      id: field.id,
      validated: isValid,
      rule: field.rule
    });
    if (isValid) {
      fieldsResults.valid += 1;
    } else {
      fieldsResults.invalid += 1;
    }
  }

  fieldsResults.allValid = fieldsResults.invalid === 0;
  return fieldsResults;
};

/**
 * Given a rule and a value, validate the value.
 *
 * @param {String} ruleName - A rule name used to validate the value against.
 * @param {String} value - The value to check.
 * @returns {boolean} Returns a Boolean value that indicate whether or not a value passes a rule check.
 */
const validateRule = (ruleName, value) => {
  switch (ruleName) {
    case "not-empty":
      return String(value).trim() !== "";
    case "email-address":
      const emailRegex = /^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/;
      return emailRegex.test(value);
    case "phone-number":
      const phoneRegex = /^[0-9]{3}-[0-9]{3}-[0-9]{4}$/;
      return phoneRegex.test(value);
    default:
      return false;
  }
};